1. Field of the Invention
The present invention relates to induction melting crucibles generally called as segmented cold wall induction melting crucible having construction suitable for melting metals and alloys called special metals mentioned below, and more particularly to improved crucible body construction including side wall and base portion.
The special metals referred to above can be classified into the following three types, that is,
1) high purity metals and alloys known to be semi-conductor materials,
2) reactive metals such as titanium, zirconium(Zr) and their alloys which are difficult to melt as high purity products due to their chemical properties being liable to react with oxygen and nitrogen, and
3) metals having very high melting temperature such as tungsten(W), molybdenum(Mo), tantalum and niobium.
2. Discussion of the Prior Art
Heretofore, special metals and alloys referred to above have been melted using electron beam melting furnace, a nonconsumable electrode arc furnace or the like, however these furnaces have revealed some problems in melting the above-mentioned high purity or reactive metals and alloys which are difficult to melt, for instance, with an electron beam melting furnace it is necessary to restrict its melting atmosphere so as to be lower than 10.sup.3 Torr, and thus induction melting techniques have been widely used in place of the electron-beam melting by virtue of the simple construction requested together with covenient ways of use.
Among various induction melting furnaces, cold wall induction melting (alternately referred to as cold-crucible induction melting,induction skull melting or the like) has been widely adopted as a melting method suitable for melting the aforesaid special metals and alloys.
A cold wall induction crucible which has generally been used heretofore, as typically shown in FIGS. 7A and 7B, is a metal crucible used for induction melting comprising; a main crucible body 1 made of metals having good thermal and electrical conductivity mainly of copper and formed to have a shape generally of hollow cylinder with a bottom, the side wall of which or even the entire side wall and a part of the base portion being divided into a plurality of segments 3 by a plurality of slits 2 separating adjacent segments 3, and an induction heating coil 8 surrounding the crucible main 1, wherein the interior of each segment 3 is cooled by a cooling medium such as water.
Next, an explanation will be provided as to the process of melting the metallic material charged in the cold wall type induction melting crucible.
A metal or metals and or master alloys to be melted are charged into the crucible chamber in the form of lumps, granules, plate-like, powder mixtures thereof or the like.
Upon starting of induction melting, the charged materials start melting beginning from the outer surface thereof and the molten metal flows down toward the bottom of the crucible, where it solidifies as a skull of a shallow dish-like configuration acting as a second metellurgical vessel holding thereon the molten metal and the charge not yet melted.
As the amount of molten metal increases due partly to stirring caused by the rather high frquency of the power than that supplied to ordinary induction melting process, the level of the molten metal rises and the skull formed around the inner surface of the segments also grows upward and forms a side wall of the skull upstanding integral with the dish-like bottom part and constitutes a metallurgical vessel like a pan, which prevents molten metel from directly adhering to the side wall of the segments and entering into the slits and contains therein the increased amount of molten metal and the charge still remaining unmelted.
As melting further proceeds until the whole of the charge has melted down, the level of the molten metal also rises so as to constitute a molten pool of the charged metal.
Under this condition, central part 9a of the molten metal 9 along the vertical axis of the crucible body is raised upward, while the peripheral portion of the molten metal 9 is lowered along the side wall of the crucible to form a convex curved surface as shown in FIG. 7A, thereby the molten metal generally leans away from contact with the inner face of the segments 3, thereby preventing any of the adjacent water cooled segments 3 electrically insulated from each other by respective slits from being shorted by the molten metal.
Copper is selected as the material suitable for forming a crucible in place of a usual refractory material. By virtue of its high thermal and electrical conductivity, segments made of copper forming a side wall of the crucible when cooled by a suitable cooling medium such as water, can be held at a temperature considerably lower than that of the molten metal received therein.
Since the crucible is constructed as explained above, the crucible itself will never reach such a high temperature at which it reacts with the metal or metals to be melted so that undesirable chemical compounds are formed which would contaminate the molten metal as usually encountered in crucibles made of refractory material which are impossible to water cool.
Instead, since the substances to be melted are limited only to those materials charged into the crucible, inclusion of impurities can be avoided thereby giving rise to high purity products, and moreover the uniformity in the melting process also can be attained by virtue of a stirring action which is a feature of induction melting.
On the other hand, due to the high electrical conductivity there is such another problem that the crucible itself is liable to be induction heated, but the possibity of the crucible being heated as a secondary coil with respect to a primary induction coil up to such a high temperature as mentioned above, can be avoided by splitting the crucible wall vertically into a plurality of segments each having therein a hollow cylindrical chamber into which a cooling medium such as water is introduced for cooling each segment.
However, it is still inevitable for the segments, due to their good electrical conductive properties, to be heated to some extent by the primary induction heating coil in a manner similar to the material to be melted.
Actual measurements have revealed that, if the amount of power used for melting the material charge is assumed to be a value of one (1) the power used for heating the crucible was found 1,3, and as result it has been roughly estimated that only 40% of the input power was utilized for melting charged material.
It is considered effective, as a measure for reducing the extent of heating for the water cooled segments themselves, to reduce the radial wall thickness of each segment, however, each water cooled segment of conventional type, as shown in FIGS. 7A and 7B, is a double tube construction having both an inlet passage and an outlet passage in a single segment, that is, cylindrical hallow tube 11 further receives therein an inner tube 11a, and as a consequence there is a restriction for reducing the radial thickness of each segment in the radial direction.
On the other hand, the greater the number of segments disposed, the less becomes the chance that each segment is heated, and the magnetic flux density adjacent to the wall of the crucible becomes more uniform which contributes to stabilize the molten metal, however, there still remains the same restriction as mentioned above due to the restriction for reducing the wall thickness in the radial direction.